The present invention relates to hybrid circuits generally, and more specifically to hybrid circuits formed using low temperature cofired ceramic materials.
Multilayer substrates for hybrid microcircuits may be advantageously fabricated using a low temperature cofired ceramic process such as the process described in U.S. Pat. No. 5,176,771, which is incorporated by reference herein. This technology utilizes dielectric sheets in the form of low-temperature-cofired-ceramic (LTCC) tape. The tape contains a material such as a mixture of glass and ceramic fillers, which sinter at about 850xc2x0 C., and exhibits thermal expansion similar to Alumina. The LTCC tape sheets are metallized to make a ground plane, signal plane, bonding plane, or the like, or they may be formed with vias, which are filled with metallizations to form interconnect layers. The sheets of tape are stacked on each other, laminated together at a relatively low laminating temperature and pressure, and then fired to sinter the ceramic material in the tape.
As power consumption and circuit density in hybrid circuits increases, it has become necessary in some instances to provide embedded cooling channels or cavities in the substrate. These channels may conduct a cooling gas or, for some high-power or high density circuits, a cooling liquid. The ability to form embedded channels in LTCC substrates is important for emerging miniaturized electronics systems.
The process for fabricating embedded open cavities in low temperature co-fire ceramic substrates has been very difficult. Prior efforts included multi-step lamination build-ups with specially-cut mylar stabilizers and inserts that needed to be removed before sintering the substrate.
Cavity feature geometries were very difficult to preserve through the lamination and firing process. These structures are susceptible to plastic deformation upon lamination or under the stress of body forces (e.g., gravity) when the glass transition temperature of the glass binder is reached. Open cavities tended to sag or collapse during lamination and sintering. When an LTCC tape with holes or channels having dimensions over 400 micrometers is laminated, the tapes above and below the holes or channels deform in the inside of the holes or channels.
Previous solutions to the above problems required exotic materials and expensive SLA equipment [What does SLA stand for?] to fabricate preforms.
For example, M. R. Gongora-Rubio et al., xe2x80x9cOverview of Low Temperature Co-Fired Ceramics Tape Technology for Meso-System Technology (MsST)xe2x80x9d, Sensors and Actuators A 89 (2001), pp 222-241, sugggests the use of fugitive phase materials intended to disappear during firing for supporting bridging structures. As an example, carbon black is suggested as a fugitive phase. Sintering is accomplished in a neutral or just slightly oxidizing atmosphere. The gasification of the carbon (reaction with oxygen to form carbon mono, and dioxide) is slow and little carbon black is lost before the bridging or suspended ceramic structure becomes rigid. After that point, one can open the furnace to air and burn-off the carbon black. The author describes that after the carbon black gasification and sintering, the upper and lower layer in the cavity filled with the carbon black-binder mixture are parallel.
One of the problems with the technique described by Gongora-Rubio et al. is that the carbon black is not evacuated completely from the cavity. Carbon black and or oxidation products thereof may remain in the cavity after firing is completed. Further, this method requires close control of the atmosphere during firing. Also, the carbon black may react with the LTCC material.
In another technique, Mylar or rubber inserts were inserted in the channels before lamination, and were mechanically pulled through holes in the substrate after lamination. Removal of the inserts is difficult and time consuming.
The prior art channel fabrication processes for LTCC devices are not compliant with current substrate fabrication techniques, or sufficiently versatile for arbitrary channel geometries. Further, the prior art techniques have high cost and are not sufficiently reliable for accurate feature preservation.
Other package cooling mechanisms have their disadvantages. Surface-mount cooling devices (sinks, fans and the like) are not volume efficient, and are inadequate for the next generation of miniaturized systems. Silicon micro-electro-mechanical systems (MEMS) based cooling systems use too much footprint on the substrate, and are not compliant with preferred electronic packaging techniques.